1. Field of the Invention
This invention relates to a slipping clutch for the torque-limiting transmission of force between a spool core of a spool for the winding and unwinding of a tape and a rotary support for said spool core.
2. Discussion of the Related Art
Slipping clutches of the type in question are widely used, above all in accessories for the home and office, for example for adhesive tape dispensers or for transfer dispensers. Transfer dispensers are hand-held devices for transferring a film, for example an adhesive film or a cover-up film, from a carrier tape to a substrate, the carrier tape being wound onto a feed spool inside the dispenser and being guided from the feed spool to a so-called applicator foot projecting from the dispenser where it reverses direction and returns to a take-up spool inside the dispenser. The film-coated carrier tape can be pressed onto a substrate via the applicator foot so that the film layer is separated from the carrier tape in conjunction with the reversal of direction of the tape and transferred to the substrate while the empty carrier tape is returned to the take-up spool inside the dispenser. The feed spool, take-up spool and applicator foot are often combined in a cassette. The feed spool and take-up spool may be arranged coaxially of or axially parallel to one another in the housing of the dispenser.
The slipping clutches mentioned above are used either in transfer dispensers to drive the take-up spool from the feed spool, which is driven by take-off of the tape, with the slipping clutch in between. The slipping clutch compensates for the equalization of speed between the feed spool and the take-up spool which is necessary for building up the tape tension that must be maintained. However, slipping clutches may also be used in dispensers where only one spool (i.e. the feed spool) is designed to rotate freely in the housing, the user offwinding the tape (for example an adhesive tape) as required for removal, to provide the freely rotating spool with a required brake torque in order to maintain a certain tape tension, for example during offwinding of the tape (i.e. dispensing of the tape for wrapping purposes), or even to prevent the feed spool from unwinding too quickly in the event of a slightly stronger pull by the user or from undesirably overrotating at the end of the removal process.
In known dispensers, slipping clutches of the type in question assume different forms. In the dispenser disclosed in DE-A-40 34 145, the feed spool and the take-up spool are connected to one another by a flat belt drive which travels over belt pulleys and which acts solely by friction. The tension of the belt drive is adapted to the required slipping torque, the slip coming in when the brake force determining the tension required for the carrier tape is reached at the take-up spool.
DE-A-39 00 156 describes a slipping clutch which operates on the principle of a magnetic clutch and in which a magnetic disc is used on the drive side and a steel disc on the driven side. The slipping torque is determined by the attractive force between the magnetic disc and the steel disc and the resulting friction.
According to EP-A-0 556 406, the take-up spool is press-fitted via a friction sleeve onto a shaft driven directly from the feed spool via a gear. The contact pressures or rather forces are selected so that slip comes in when the required tape tension is reached or when the corresponding brake torque is generated at the take-up spool.
U.S. Pat. No. 4,891,090 describes another slipping clutch in which one part of the clutch in the form of a shaped element is provided with several resilient plastic arms arranged in the shape of a star which press against and engage by friction with the inner wall of another part of the clutch in the form of an annular sleeve. The contact pressure of the arms and the friction pairing of their constituent material and that of the annular sleeve determine the transmittable torque or rather the slipping moment.
All the slipping clutches mentioned are designed in principle as friction clutches so that the magnitude of the slipping moment is always determined by the actual frictional resistance between the parts rubbing against one another. In the case of transfer dispensers, the tape resistance at which slip occurs must be at least slightly greater than the force which is necessary to guide the carrier tape over the applicator element under the requisite tension, to separate the adhesive layer applied and to rewind the carrier tape onto the take-up spool. On the other hand, however, the torque at which slip occurs must not exceed the cohesive force of the coating (for example adhesive layer) applied to the carrier tape because this would lead to tearing of the film so that transport of the tape would stop and blockage would ultimately occur. In addition, the leverage acting on the take-up spool would increase with increasing wind-on radius so that the slip of the clutch would have to be gauged in such a way that even the final section of the carrier tape to be wound onto the take-up spool could still be safely wound on.
In the known slipping clutches, the main influencing factor is the frictional force between the workpieces rubbing against one another. It is dependent to a large extent, for example, on the abrasion behavior or behavior under pressure of the parts sliding on one another. Even if only minimal abrasion occurs in the practical application of a dispenser, the contact pressure applied to the friction surface will diminish considerably unless the various parts are under spring tension, with the result that the friction force and the hence the slipping moment will also decrease. However, if resilient, for example elastically deformed, parts are used to apply pressure to the parts rubbing against one another, as is the case with the dispenser known from U.S. Pat. No. 4,891,090, the level of tension will fall to a certain extent over the life of the dispenser due to the viscoelastic behavior of the plastics used for the resilient parts. The loading of permanently resiliently deformed plastic springs should be comparatively small to prevent them from being overly permanently deformed with time. So far as the slipping clutches in question are concerned, however, this means that they are capable for a limited time of transmitting the torque for which they were designed and which is required for the smooth operation of a transfer dispenser or the like. This, in conjunction with the brake torque likewise increasing with increasing winding radius of the take-up spool, means that, in order to ensure winding up to the end of a tape, a comparatively large slipping moment has to be taken into account at the beginning of the tape's life in order to guarantee sufficient pull when the take-up spool is almost completely full at the end of the tape's useful life.